Method of controlling an automotive vehicle having a trailer using rear axle slip angle

ABSTRACT

A system and method of controlling a vehicle with a trailer comprises determining the presence of a trailer, determining a vehicle velocity, determining a steering wheel angle, and determining a rear axle side slip angle. When the rear axle side slip angle is above a predetermined value, the vehicle velocity is above a velocity threshold, and the steering wheel is about zero, brake-steer is applied to the vehicle.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is related to U.S. Applications (Attorney DocketNo. 81093807/FGT-1902 PA) entitled “Control System for Brake-SteerAssisted Parking and Method Therefor”; (Attorney Docket No.81093810/FGT-1903 PA) entitled “Method and Apparatus of Controlling anAutomotive Vehicle Using Brake-Steer as a Function of Steering WheelTorque”; (Attorney Docket No. 81093816/FGT-1904 PA) entitled “Method andApparatus for Controlling an Automotive Vehicle Using Brake-Steer andNormal Load”; (Attorney Docket No. 81093819/FGT-1905 PA) entitled“Method and Apparatus for Controlling Brake-Steer in an AutomotiveVehicle in Reverse”; (Attorney Docket No. 81093821/FGT-1906 PA) entitled“Method and Apparatus for Controlling Brake-Steer in an AutomotiveVehicle in a Forward and Reverse Direction”; (Attorney Docket No.81093822/FGT-1907 PA) entitled “Method of Controlling an AutomotiveVehicle Having a Trailer”; (Attorney Docket No. 81093839/FGT-1909 PA)entitled “Method and Apparatus for Maintaining a Trailer in a StraightPosition Relative to the Vehicle”; (Attorney Docket No.81093840/FGT-1910 PA) entitled “Method and Apparatus for Predicting thePosition of a Trailer Relative to a Vehicle”; (Attorney Docket No.81093841/FGT-1911 PA) entitled “Method and Apparatus for Controlling anAutomotive Vehicle in a U-Turn”; (Attorney Docket No. 81093842/FGT-1912PA) entitled “Method and Apparatus to Enhance Brake-Steer of a VehicleUsing a Controllable Suspension Component”; (Attorney Docket No.81093843/FGT-1913 PA) entitled “Method and Apparatus for Controlling aVehicle Using an Object Detection System and Brake-Steer”; (AttorneyDocket No. 81093849/FGT-1916 PA) entitled “Method and Apparatus forControlling a Trailer and an Automotive Vehicle With a Yaw StabilityControl System”, each incorporated by reference herein.

BACKGROUND OF INVENTION

The present invention relates generally to a dynamic control system foran automotive vehicle, and more particularly, to a system forcontrolling brake-steer in the presence of a trailer using a rear axleside slip angle.

Dynamic control systems for automotive vehicles have recently begun tobe offered on various products. Dynamic control systems typicallycontrol the yaw of the vehicle by controlling the braking effort at thevarious wheels of the vehicle. Yaw control systems typically compare thedesired direction of the vehicle based upon the steering wheel angle andthe direction of travel. By regulating the amount of braking at eachcorner of the vehicle, the desired direction of travel may bemaintained.

Such systems typically include the capability of controlling one wheelor multiple wheels individually. That is, the vehicle wheels may bebraked individually. Individual braking is typically performed on ademand basis for a relatively short time to stabilize the vehicle.Further, a vehicle wheel may be provided with a different torque thanthe other wheels. This may be desirable to perform certain controls indynamic stability control systems.

Large vehicles such as full-size sport utility vehicles, pickup trucks,and heavy duty trucks have a large turning radius. Such vehicles may beused to pull trailers. It would be desirable to improve the turningcharacteristics of these vehicles by reducing the turning radius. Itwould also be desirable to improve the trailering characteristics of avehicle.

One system that is known to improve the turning characteristics of thevehicle is a four wheel steer system. By steering the rear wheels in theopposite direction of the front wheels in low speed, the turning radiusof the vehicle is reduced. Four wheel steering is also capable ofimproving the trailerability of a vehicle in high speed. One drawback tosuch a system is that the system adds another steering actuator to thevehicle. This increases the cost, complexity, warranty, maintenancecosts and weight of the vehicle. In contrast, it is typically theobjective today to reduce the cost and weight of vehicles.

It would therefore be desirable to improve the turning capability andtrailerability of vehicles without incurring the drawbacks of a fourwheel steering system.

SUMMARY OF INVENTION

The present invention improves the trailerability of a vehicleparticularly at high speeds by monitoring and controlling the rear axleside slip angle of the vehicle. By using controlled brake systems tocontrol the rear axle side slip angle of the vehicle, the lateralvariance of the trailer may be controlled, especially during a highspeed driving when the potential divergent lateral motion of the trailercould be excited.

In one aspect of the invention, a method of controlling a vehicle with atrailer comprises determining a presence of a trailer, determining avehicle velocity, determining a steering wheel angle, and determining arear axle side slip angle. When the rear axle side slip angle is above apredetermined value together with rate change of the side slip angleabove certain threshold (indicating that the side slip angle isconstantly crossing zero), the vehicle velocity is above a velocitythreshold, and the steering wheel angle is about zero, the brake-steersystem determines that the vehicle is in straight line driving and thetrailering is potentially unstable, brake-steer is applied to thevehicle. The controlled brake torque may be set to be proportional tothe magnitude of the rear axle side slip angle and/or the magnitude ofthe rate change of the rear axle side slip angle.

In straight line driving, the vehicle yaw stability control usually doesnot activate. Hence the brake-steer action for straight line andunstable trailering is an addition to the yaw stability control.

In a further aspect of the invention, during a turning, the brake-steeris activated upon the detection of a potentially unstable traileringcondition.

The yaw stability control tuned for normal vehicle driving is based onthe driver's intention to control the over-steer or under-steer of thevehicle such that the vehicle is maintained on the driver's intendedcourse. Since yaw error feedback and side slip feedback are used in yawstability control, problems may be caused during turning and unstabletrailering. In one aspect, the divergent trailer lateral motion wouldcause a fluctuation of the vehicle yaw rate and side slip angle. Fromyaw stability control point of view, the vehicle crosses the over-steerand under-steer boundary from time to time. Hence under-steer correction(attempting to make car steer more) and over-steer control (attemptingto make car steer less) will activate. If those activations are notcarefully done, excitation of the trailer's dynamic lateral deviationinstead of controlling the trailering may be initiated.

Therefore it may desirable to provide a control system to enhance thetraditional yaw stability control upon the detection of the unstabletrailering during a turning. Such a system uses the braking to steer thevehicle in an opposite direction of the trailer motion so as tostabilize the trailering. Such a system includes determining a presenceof a trailer, determining a vehicle velocity, determining a hand wheelangle position signal of the hand wheel, determining a sensor yaw ratefrom the yaw rate sensor, calculating a desired yaw rate based upon thehand wheel angle position signal (which reflects the driver'sintention), determining a rear axle side slip angle, and controlling thebrakes of the vehicle such that the vehicle unstable trailering iseliminated.

More specifically, upon the detection of the trailering in a turnedvehicle, when the rear axle side slip angle is determined to be above apredetermined rear axle slip with rate change of the side slip angleabove certain threshold, and the vehicle velocity is above a vehiclevelocity threshold, one or more brake control commands (amount of thebrake pressures) are generated based on the magnitude of the calculatedrear side slip angle and the magnitude of its rate change, the yawangular rate, the desired yaw angular rate.

The location of brakes in which the braking pressures are sent isdetermined based on the practice of reducing the magnitude of the rearside slip angle. That is, when there is a positive rear side slip angle,the braking is applied to a wheel such that the vehicle intends togenerate negative side slip angle upon the application of braking; whenthere is a negative rear side slip angle, the braking is applied to awheel such that the vehicle intends to generate positive rear side slipangle upon the application of braking.

One advantage of the invention is that high speed tracking of thetrailer behind the vehicle may be controlled using brake-steer. Thepresent invention may be formed to be non-intrusive and may beinexpensively implemented.

Other advantages and features of the present invention will becomeapparent when viewed in light of the detailed description of thepreferred embodiment when taken in conjunction with the attacheddrawings and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is plot of a vehicle traveling along three curves correspondingto a conventional vehicle and two embodiments of the invention.

FIG. 2 is a perspective view of an automotive vehicle on a road surfacehaving a control system according to the present invention.

FIG. 3 is a block diagrammatic view of a control system according to thepresent invention.

FIG. 4 is a high level block diagrammatic view of systems of theautomotive vehicle according to the present invention.

FIG. 5 is a perspective view of a trailer locating plate coupled to atrailer tongue relative to the vehicle.

FIG. 5A is a perspective view of the toe locating plate of FIG. 5.

FIG. 5B is a top view of an apparatus for determining the position ofthe trailer.

FIG. 6 is a perspective view of a Hotchkiss suspension having an activecompliant suspension component according to the present invention.

FIG. 7 is a perspective view of an independent suspension.

FIG. 8 is an exploded view of a tow link of an independent suspension.

FIG. 9 is a simplified view of an electric vehicle that may usebrake-steer according to the present invention.

FIG. 10 is a flow chart of a first embodiment of the present invention.

FIG. 11 is a plot of various boost curves relative to an amount of brakesteering wheel angle dependent upon steering wheel rate or torque.

FIG. 12 is a plot of various boost curves relative to an amount oftorque.

FIG. 13 is a plot of a boost curve used from a vehicle V=0 to a lowvelocity threshold.

FIG. 14 is a second embodiment plot of a boost curve used from V=0 to alow velocity threshold.

FIG. 15 is a flow chart illustrating a method of operating a secondembodiment of the present invention.

FIG. 16 is a simplified top view of a vehicle having a controllablesuspension illustrating brake-steer in a forward direction.

FIG. 17 is a simplified top view of a vehicle having a controllablesuspension illustrating brake-steer in a reverse direction.

FIG. 18 is a simplified top view of a vehicle having a controllablesuspension illustrating brake-steer on a split mu surface.

FIG. 19 is a block diagrammatic view of a third embodiment of thepresent invention.

FIG. 20 is a plot of a screen display of a trailer and vehicle withpredicted positions.

FIG. 21 is a flow chart of a fourth embodiment of the present invention.

FIG. 22 is a flow chart of a fifth embodiment of the present invention.

DETAILED DESCRIPTION

In the following figures the same reference numerals will be used toidentify the same components. The various terms and values are set forthby way of example and are not meant to be limiting unless specificallyset forth in a claim.

Referring now to FIG. 1, a vehicle 10 is illustrated traversing threepaths. Path Al is the path a vehicle travels without the invention. PathA2 is a path the vehicle 10 travels with brake-steer. Path A3 is a paththe vehicle 10 travels with brake-steer and a controllable suspensioncomponent. As is shown, path A2 improves the turning radius over pathAl. Path A3 has a reduced or improved turning radius compared to pathA2.

The term brake-steer or brake-steering is used to describe changing acharacteristic of the vehicle such as the turning radius or tracking ofthe vehicle using one or more brakes, the application of differential(positive or negative) torques, or a combination of the braking anddifferential torques. Positive torques may be applied by use of electricdrive motors (with or without an electric vehicle), activedifferentials, or traditional torque distribution methods. Activedifferentials are capable of diverting all or part of the drive torqueto one side of the vehicle or the other. Also, specific configurationsmay depend on different vehicle arrangements including powertrain,weight, loading, tires and the other desired effects. Vehicles employingsuch systems will be calibrated and/or adjusted experimentally. Thepresent invention is particularly suitable for use in long wheel basevehicles. However, shorter wheel base vehicles may also benefit fromimplementation of this invention.

The present invention may be used with various dynamic control systemssuch as, but not limited to, anti-lock brakes, traction control, rollstability control and yaw control systems. The present invention isdiscussed below in terms of preferred embodiments relating to anautomotive vehicle moving in a three-dimensional road terrain, but it isto be understood that such descriptors are not to be limiting on thefull range and scope of the present invention. Further, the varioussensors may be used alone or in various combinations depending on theconditions. Other sensors may be used to complement or verifydeterminations of other sensors. For example, some sensors may be usedto check the image or radar signals, or vice versa.

Referring to FIG. 2, an automotive vehicle 10 with a control system ofthe present invention is illustrated. Vehicle 10 has front right andfront left tires 12 a and 12 b and rear right tires 13 a and rear lefttires 13 a and 13 b, respectively. The vehicle 10 may also have a numberof different types of front steering systems 14 a including having eachof the front wheels configured with a respective controllable actuatorsand the front wheels having a conventional type system in which both ofthe front wheels are controlled together. The vehicle has a rear axlesystem 14 b. Generally, the vehicle has a weight represented as Mg atthe center of gravity of the vehicle, where g =9.8 m/s² and M is thetotal mass of the vehicle.

The sensing system 16 may share sensors with other vehicle dynamiccontrol systems such as a yaw stability control system sensor set or aroll stability control system sensor set. Of course, the actual sensorsused will vary depending on the type of control system or systemsimplemented on the particular vehicle. The various possible sensors willbe further described below. The wheel speed sensors 20 may be mounted asadjacent each wheel of the vehicle. Those skilled in the art willrecognize three wheel speed sensors may be used. For example, one forthe rear of the vehicle and one for each of the front two wheels. Theremaining sensors of sensing system 16 are preferably mounted directlyat the center of gravity of the vehicle, along the reference directionsx, y and z shown in FIG. 1. As those skilled in the art will recognize,the frame from b₁, b₂ and b₃ is called a body reference frame 22, whoseorigin is located at the center of gravity of the car body, with the b₁corresponding to the x axis pointing forward, b₂ corresponding to the yaxis pointing off the left side, and the b₃ corresponding to the z axispointing upward. The angular rates of the car body are denoted abouttheir respective axes as ω_(x) for the roll rate, ω_(y) for the pitchrate, and ω_(z) for the yaw rate. The present invention calculationspreferably take place in an inertial frame 24 that may be derived fromthe body reference frame 22 as described below.

As will be described below, the sensing system 16 may also include alidar, radar and/or sonar sensor(s), camera(s), a GPS system and variousother sensors (all of which are shown in FIG. 2 or 3 below).

The angular rate sensors and the accelerometers are mounted on thevehicle along the body frame directions b₁, b₂ and b₃, which are the xaxes of the vehicle's sprung mass.

The longitudinal acceleration sensor is mounted on the vehicle locatedat the center of gravity, with its sensing direction along the b₁-axis,whose output is denoted as a_(x). The lateral acceleration sensor ismounted on the car body located at the center of gravity, with itssensing direction along b₂-axis, whose output is denoted as a_(y). Thevertical acceleration sensor is mounted on the car body located at thecenter of gravity, with its sensing direction along b₃-axis, whoseoutput is denoted as a_(z).

The other reference frames used in the following discussion includes theroad reference frame, as depicted in FIG. 2. The road reference framesystem r₁r₂r₃ is fixed on the driven road surface at any instant intravel time of the vehicle, where the r₃ axis is along the average roadnormal direction computed from the normal directions of thefour-tire/road contact patches.

In the following discussion, the Euler angles of the body frame b₁b₂b₃with respect to the road frame r₁r₂r₃ are denoted as θ_(xbr), θ_(ybr)and θ_(zbr), which are also called the relative Euler angles.

Referring now to FIG. 3, control system 18 is illustrated in furtherdetail having a controller 26. Controller 26 in this case may be asingle centralized vehicle controller or a combination of controllers.If many controllers are used they may be coupled together to communicatevarious information therebetween, and arbitration and prioritizationamong multiple controllers might also be performed. Preferably, thecontroller 26 is microprocessor-based.

The controller 26 may be programmed to perform various functions andcontrol various outputs. Controller 26 may also have a memory 27associated therewith. Memory 27 may be a stand-alone memory or may beincorporated within the controller 26. Memory 27 may store variousparameters, thresholds, patterns, tables or maps. For example, a map ofhow much brake-steer to generate in response to steering wheel rate andvehicle velocity may be stored in memory. Such maps may be calibratableduring vehicle development.

The controller 26 is used for receiving information from a number ofsensors, which may include speed sensors 20, a yaw rate sensor 28, alateral acceleration sensor 32, a roll rate sensor 34, a verticalacceleration sensor 35, a longitudinal acceleration sensor 36, a pitchrate sensor 37, and steering angle position sensor 38. Sensors 28-38 maybe part of an inertial measurement unit 40 or IMU.

In one embodiment, the sensors 28-37 are located at the center ofgravity of the vehicle. Those skilled in the art will recognize that thesensors may also be located on various locations off the center ofgravity and mathematically translated equivalently thereto.

Roll rate sensor 34 and pitch rate sensor 37 may be used to sense thevehicle roll and pitch conditions. The roll and pitch conditions of thevehicle might be conducted based on sensing the height of one or morepoints on the vehicle relative to the road surface. Sensors that may beused to achieve this include a radar-based proximity sensor, alaser-based proximity sensor and a sonar-based proximity sensor.

Roll and pitch conditions of the vehicle may also be sensed based onsensing the linear or rotational relative displacement or displacementvelocity of one or more of the suspension chassis components which mayinclude a linear height or travel sensor, a rotary height or travelsensor, a wheel speed sensor used to look for a change in velocity, asteering wheel position sensor, a steering wheel velocity sensor and adriver heading command input from an electronic component that mayinclude steer by wire using a hand wheel or joy stick.

The roll and pitch conditions may also be sensed by sensing the force ortorque associated with the loading condition of one or more suspensionor chassis components including a pressure transducer in an active airsuspension, a shock absorber sensor such as a load cell, a strain gauge,the steering system absolute or relative motor load, the steering systemassist pressure, a tire laterally force sensor or sensors, alongitudinal tire force sensor, a vertical tire force sensor or a tiresidewall torsion sensor.

The roll and pitch condition of the vehicle may also be established byone or more of the following translational or rotational positions,velocities or accelerations of the vehicle including a roll gyro, theroll rate sensor 34, the yaw rate sensor 28, the lateral accelerationsensor 32, a vertical acceleration sensor 35, a vehicle longitudinalacceleration sensor 36, lateral or vertical speed sensors including awheel-based speed sensor, a radar-based speed sensor, a sonar-basedspeed sensor, a laser-based speed sensor or an optical-based speedsensor.

Lateral acceleration, roll and pitch orientations and velocities may beobtained using a global positioning system (GPS) 41.

The controller 26 may also be coupled to a lidar, radar, or sonar 42.The lidar, radar, or sonar 42 may be used to generate a velocity signalor relative velocity signal of an object. The radar or lidar may also beused to generate a trajectory signal of an object. Likewise, thevelocity of the vehicle in various directions may be obtained relativeto a stationary object. A lidar, radar, or sonar sensor 42 may bemounted in various positions around the vehicle including the front,sides and/or rear. Multiple sensors 42 may also be employed in multiplelocations to provide multiple information from multiple positions of thevehicle. Such signals may also be used in a self parking condition.

Controller 26 may also be coupled to a camera system 83 having cameras43 a-43 e. A stereo pair of cameras 43 a, 43 b may be mounted on thefront of the vehicle to detect target objects in front of the vehicle,to measure the object size, range and relative velocity and to classifythose objects into appropriate categories. Camera 43 c may be mounted onthe right side of the vehicle, camera 43 d may be mounted on the leftside of the vehicle, and camera 43 e may be directed rearward of thevehicle. Camera 43 e may also include a stereo pair of cameras. All orsome of the cameras may be used in a commercial embodiment. Also, astereo pair of cameras 43 a, 43 b may be replaced by a single camera (43a or 43 b) depending on the roll and pitch conditions measured by thesystem. Various types of cameras would be evident to those skilled inthe art. Various types of cameras such as a CMOS-type camera or aCCD-type camera may be implemented to generate various image signals. Aswill be further described below, the various image signals may beanalyzed to determine the various dynamic conditions of the vehicle.

Controller 26 may also be coupled to an input device 44. Input device 44may include a keyboard or other push button type device. Input device 44may be used to enter trailer parameters or indicate to the controller aselection or other inputs.

A reverse aid system 46 having at least one reverse aid sensor 48 may becoupled to controller 26. Reverse aid sensor 48 may be but is notlimited to an ultrasonic sensor, a radar sensor, or a combination of thetwo. Reverse aid sensors 48 are typically located at several locationsof the rear of the vehicle such as in the bumper. As will be furtherdescribed below, the reverse aid system 46 may be used to provide anindication as to the presence of a trailer and may also be used togenerate a particular pattern with respect to the trailer to allow thecontroller to have feedback with respect to the position of the trailer.

A hand wheel (also known as “steering wheel”) position sensor 50 mayalso be coupled to controller 26. Hand wheel position sensor 50 providescontroller 26 with a signal corresponding to the relative rotationalposition of the steering wheel within the vehicle. Various types ofsensors include absolute sensors and position sensors using a centerfind algorithm (relative sensors). Relative sensors may use thecenterfind algorithm to determine the position relative to a centerposition once the position is known. Both types of sensors may provide asteering angle rate signal and/or a steering direction signal. Forexample, the steering direction may indicate away from or toward acenter position or end stop position.

A hand wheel torque sensor 52 may also be coupled to controller 26. Handwheel torque sensor 52 may be a sensor located within the steeringcolumn for direct measurement. The steering torque may also be inferredfrom data available to the power steering system. The hand wheel torquesensor 52 generates a signal corresponding to the amount of torqueplaced on the hand wheel (steering wheel within the vehicle).

A mu (μ) sensor 54 may also be coupled to controller 26. Mu sensor 54may be a direct sensor or, more likely, is a calculated value based onavailable inputs. Various systems such as a yaw control system for ananti-lock brake system may generate mu. Mu is an indication of thecoefficient of friction of the surface on which the vehicle istraveling. The mu sensor 54 may be used to generate a coefficient offriction for the vehicle or the coefficient of friction at more than onecontact patch of the tire. Preferably, a mu is determined at eachcontact patch of each tire.

A throttle sensor 56 may also be coupled to controller 26. Throttlesensor 56 may, for example, be a resistive sensor. Of course, othertypes of throttle sensors would be evident to those skilled in the art.Throttle sensor 56 generates a signal corresponding to the position ofthe throttle of the vehicle. The throttle sensor 56 may give anindication as to the driver's intention regarding acceleration. Throttlesensor may also be part of a drive-by-wire type system. A throttle typesensor may also be used in electric vehicles and vehicles with dieselengines to determine the desire acceleration. These sensors may take theform of a pedal sensor.

A vehicle load sensor 58 to sense the amount of weight or payload withinthe vehicle may also be coupled to controller 26. Vehicle load sensor 58may be one of various types of sensors including a suspension sensor.For example, one load sensor may be located at each suspensioncomponent. Load sensor 58 may, for example, be a pressure sensor in anair suspension. The load sensor 58 may also be a load cell. In any case,the vehicle load sensor 58 generates an electrical signal correspondingto the load on the vehicle. One sensor or preferably one sensor for eachcorner of the vehicle may be used. The vehicle load may, for example, bethe normal load at each corner of the vehicle. By knowing the normalload at each corner of the vehicle, the total amount of loading on thevehicle may be determined.

A suspension height sensor 60 may also be coupled to controller 26.Suspension height sensor 60 may be a suspension height sensor located ateach corner of the vehicle. Suspension height sensor 60 may also be partof an air suspension or other type of active suspension. Suspensionheight sensor 60 generates a height signal corresponding to theextension of the suspension. The suspension height sensor 60 may also beused to determine the vehicle load, normal load, and payloaddistribution, rather than using vehicle load sensor 58 described above.Suspension height sensor 60 may be one of various types of sensorsincluding a laser, optical sensor, or the like.

A transmission gear selector 62 may also be coupled to controller 26.Transmission gear selector 62 may, for example, comprise a shift leverthat has the PRNDL selections corresponding to the park, reverse,neutral, regular drive and low drive positions of the transmission.Also, an electrical signal may be generated in response to the positionof the shift lever of a manual transmission.

A mode selector 64 may also be coupled to controller 26. Mode selector64 may select a driver selectable mode selector such as a manuallyactivated mechanism (e.g., push button or the like) or a voicerecognition system. Mode selector 64 may, for example, select a positionthat corresponds to trailering. Also, mode selector may determine a parkposition indicating that the vehicle operator intends to park thevehicle. A U-turn position may also be selected. The mode selector maybe used to enable or disable the system.

A secondary steering actuator 66 such as a turn signal actuator, anadditional stalk or push buttons may also be coupled to controller 26.The secondary steering actuator 66 may also initiate the display of aturn signal indicator on the instrument panel of the vehicle. Secondarysteering actuator 66 may be used to steer a trailer of the vehicle asdescribed below. For example, the vehicle or trailer may be directed ina particular direction corresponding to the secondary steering actuatordirection.

A display 68 may also be coupled to controller 26. Display 68 displaysvarious types of displays or combinations of displays. Display 68 maydisplay the various conditions of the vehicle such as the inputs fromthe input device 44, mode selector indicators from mode selector 64, andturn signal actuator 66. Display 68 may be a light on a dash panel orpart of a more complex LED or LCD display on the instrument panel of thevehicle. Of course, other locations for the display may include anoverhead display or the like. Display 68 may also be used to display theprojected position of a trailer relative to the vehicle.

Hand wheel switches 70 may be coupled to the steering or hand wheel.Hand wheel switches 70 may be labeled left and right corresponding to aleft and right direction. As will be described below, brake-steer may beinitiated in response to the switches 70. Hand wheel switches 70 mayalso be used to independently control left and right trailer brakes tohelp maneuverability of the trailer.

Based upon inputs from the sensors and/or cameras, GPS, and lidar orradar, controller 26 may control a safety device 84. Depending on thedesired sensitivity of the system and various other factors, not all thesensors 20, 28-66, cameras 43 a-43 e, lidar or radar 42, or GPS 41 maybe used in a commercial embodiment. Safety device 84 is part of avehicle subsystem control. Safety device 84 may control a passive safetydevice 86 such as an airbag, a pressure sensor 89, a steering actuator88, or a braking actuator 90 at one or more of the wheels 12 a, 12 b, 13a, 13 b of the vehicle. Engine intervention 92 may act to reduce enginepower to provide a safety function. Also, other vehicle components suchas a suspension control 94 may be used to adjust the suspension andprovide for various types of control in dynamic conditions such asbrake-steer. An anti-roll bar system 96 may be used to prevent rollover.The anti-roll bar system 96 may comprise a front or rear activeanti-roll bar, or both. It should also be noted that the systems 88-96may act alone or in various combinations. Certain systems 88-96 may actto provide a safety function when various dynamic conditions are sensed.

Steering actuator 88 may include the position of the front right wheelactuator, the front left wheel actuator, the rear left wheel actuator,and the right rear wheel actuator. As described above, two or more ofthe actuators may be simultaneously controlled. For example, in arack-and-pinion system, the two wheels coupled thereto aresimultaneously controlled.

Safety device 84 may also comprise a roll stability control system 102,an anti-lock brake system 104, a yaw stability control system 106,and/or a traction control system 108. The roll stability control system102, anti-lock brake system 104, yaw stability control system 106, andtraction control system 108 may be coupled to brake system 90. Further,these systems may also be coupled to steering actuator 88. Engineintervention 92 may also be coupled to one or more of the devices,particularly the roll stability control system, yaw stability controlsystem, and traction control system. Thus, the steering actuator 88,brake system 90, engine intervention 92, suspension control 94, andanti-roll bar system 96 may be part of one of the dynamic controlsystems 102-108. As will be further described below, the yaw stabilitycontrol system 106 may have thresholds that are set by the controller 26and that may be changed based upon the various conditions of the vehiclesuch as a trailering condition.

A warning device 112 may also be coupled to controller 26. Warningdevice 112 may warn of various conditions such as an impending rollover,understeer, oversteer, an approach of an in-path object, or impendingtrailer interference during a reverse direction. The warnings areprovided in time for the driver to take corrective or evasive action.The warning device 112 may be a visual display 114 such as warninglights or an alpha-numeric display such an LCD screen. Display 114 maybe integrated with display 68. The warning device 112 may also be anaudible display 116 such as a warning buzzer, chime or bell. The warningdevice 112 may also be a haptic warning such as a vibrating steeringwheel. Of course, a combination of audible, visual, and haptic displaymay be implemented. A blinking light or display may be used to indicatethe actual steering angle versus the steered wheel angle. That is, thelight may come on solid when the steering is enhanced by the controlsystem and blinks when less than the steering angle is beingaccomplished such as on a low mu surface.

A level-based system 118 may also be coupled to controller 26.Level-based system 118 uses the pitch level or angle of the vehicle toadjust the system. Level-based system 118 may, for example, be aheadlight adjustment system 120 or a suspension leveling system 122.Headlight adjustment system 120 adjusts the beam pattern downward for aloaded vehicle. Suspension leveling system 122 adjusts the suspension atthe various corners of the vehicle to maintain the vehicle relativelylevel to the road. The level-based system 118 may also make anadjustment based on the roll angle of the vehicle.

Referring now to FIG. 4, vehicle 10 is illustrated in further detail. Asillustrated in FIG. 1, vehicle 10 has wheels 12 a, 12 b, 13 a and 13 b.Associated with each wheel is a pair of front brakes 130 a and 130 b anda pair of rear brakes 132 a and 132 b. Brakes 130 and 132 may beindependently actuatable through a brake controller 134. Brakecontroller 134 may control the hydraulic system of the vehicle. Ofcourse, electrically actuable brakes may be used in the presentinvention. Suspension control 88 may be coupled to front adjustablesuspension components 136 a and 136 b, and rear adjustable suspensioncomponents 138 a and 138 b. The adjustable suspension components may bevarious types including magnetic field responsive fluid or anelastomeric component link or bushing. A magneto-rheological device maybe used. The components may be a link such a toe link or other controlarms of the vehicle. The adjustability may be incorporated into themounting of the suspension components such as in the bushings.

Also illustrated in FIG. 4 is front steering system 14 a described abovewith respect to FIG. 1.

Also illustrated is the reverse aid system 46 having a pair of reverseaid sensors 48 as described above.

Vehicle 10 may also have an internal combustion engine 140. Engine 140may have a throttle device 142 coupled thereto which is actuated by afoot pedal 144. Throttle device 142 may be part of a drive-by-wiresystem or by a direct mechanical linkage between pedal 144 and throttledevice 142. Engine 140 may include an engine controller 146. Enginecontroller 146 may be an independent controller or part of controller 26for the vehicle. Engine controller 146 may be used to reduce or increasethe engine power. While a conventional internal combustion engine iscalculated, the vehicle could also be powered by a diesel engine or anelectric engine or the vehicle could be a hybrid vehicle utilizing twoor more types of power systems.

A transmission 148 may be coupled to engine 140. Transmission 148 may bean automatic transmission or a manual transmission. A gear selector 150is used to select the various gears of the transmission 148. Gearselector 150 may be a shift lever used to select park, reverse, neutraland drive positions of an automatic transmission. A transmissioncontroller 152 may also be coupled to transmission 148. Transmissioncontroller 152 may be a separate component or may integrated with enginecontroller 146 or another controller such as controller 26. Both enginecontroller 146 and transmission controller 152 may be integrated aloneor together with controller 26. The various controllers may beprogrammed to perform various functions.

The output of the transmission 148 is coupled to a driveline 154. Thedriveline 154 may be coupled to a transfer case 156 having a transfercase controller 157 and a rear differential 158. In the case of anall-wheel drive vehicle, the transfer case may include a centerdifferential. Transfer case 156 may have a 4×4 mode and a 4×2 mode thatis controlled by controller 157. As will be described below, changing toa 4×2 mode from a 4×4 mode may be desirable during brake-steer. Thefront differential 156 and rear differential 158 may be closed, locking,or open differential. Various types of differentials may be useddepending on the desired vehicle performance and use. The differentialmay be controlled by controller 26. Further the controller 26 may alsoknow and/or control the operating conditions of the vehicle include 4×4mode, 4×2 mode, the locking condition of each of the differentials andhigh and low mode of a 4×4.

A trailer 160 may be towed behind vehicle 10. Trailer 160 may include atongue 161 and trailer wheels 162 a and 162 b. Of course, variousnumbers of axles/wheels may be used on a trailer having a right and leftwheel or set of wheels. Each trailer wheel 162 a, 162 b includes atrailer brake 164 a and 164 b. Trailer 160 may also include otherelectrical components such as lights 166 a and 166 b. A harness 168 maybe used to couple the electrical components such as the brakes 164 a,164 b and lights 166 a, 166 b to the vehicle 10. More precisely, theharness 168 may be used to couple the trailer to the electrical systemof the vehicle. Harness 168 may also couple the trailer 160 to a trailerbrake controller 170. Trailer brake controller 170 may be an independentcontroller or may be integrated within brake controller 134 describedabove. Preferably, trailer brake controller 170 is capable ofcontrolling brakes 164 a or 164 b together or independently. The trailer160 is coupled to the vehicle 10 through a hitch 172 located at the endof tongue 161. The hitch 172 may have a hitch sensor 174 thereon. Thehitch sensor 174 is used to determine the position of the trailerrelative to the vehicle 10. Various types of hitch sensors such asresistive, inductive, ultrasonic or capacitive type sensors may be usedto determine the relative angle of the trailer 160 with respect to thevehicle. Hitch sensor 174 may be used to determine the vehicle load.Other ways to determine the position of the trailer may include cameraslocated on either the trailer or vehicle or the reverse sensors.

Referring now to FIG. 5, a perspective view of vehicle 10 having analternative method for determining the relative position of the trailer160 relative to the vehicle is illustrated. The vehicle is illustratedhaving a ball 175 that is positioned at or near the rear bumper 176 ofthe vehicle. In this embodiment, only two reverse aid sensors 48 areillustrated. However, various numbers of reverse aid sensors may beillustrated. Trailer tongue 161 has a locating plate 177 thereon.Locating plate may, for example, have a locating hole 178 aligned withthe center of the tongue 161. In addition to or instead of locating hole178, a locating opening 179 may be positioned on the locating plate. Thelocating plate 177 is fixedly attached to the trailer or tongue 161 sothat the locating hole 178 and/or the locating opening 179 is centeredwith the tongue. The reverse sensing system detects the position ofeither the locating hole 178 or locating opening 179. Thus, the relativeposition of the trailer may be determined using the reverse aid sensors48. The reverse aid sensors 48 generate signals and locate the positionof the locating hole. The display 68 described above in FIG. 3 maygenerate a screen display or audible display based on the position ofthe locating plate and thus the tongue 161 relative to the vehicle.Thus, while backing the vehicle 10 to attached the trailer thereto, theball 175 may be more easily aligned with the trailer hitch 172. Tosummarize, a method for aligning a vehicle includes driving the vehiclein a reverse direction and sensing the position of a locating plate or alocating guide such as the hole 178 or opening 179. An indicator may begenerated in the vehicle corresponding to the position of the trailerhitch or tongue relative to the vehicle. The vehicle could beautomatically brake-steered or braked to cause alignment of the ball onthe vehicle to the hitch on the trailer.

Yet another method of determining the alignment of the trailer withrespect to the vehicle is as follows. The ball hitch 175 has a shallowsquare hole H1 at its top into which fits a mating spring-loaded rod andcorresponding spring S1 on the trailer coupler 172. (The spring-loadingprevents damage to the rod and hole if the hitch is coupled with the rodand hole out of alignment.) The rod is connected to a potentiometer P1or optical rotation sensor affixed to the trailer coupler 172. When thevehicle turns relative to the trailer, the potentiometer or opticalrotation sensor is rotated, providing a measurement of the relativevehicle-trailer angle.

Referring now to FIG. 6, a Hotchkiss rear suspension 180 is illustratedformed according to the present invention. Hotchkiss rear suspension 180may include the adjustable suspension component 138 a and/or 138 b asdescribed above. Both the front and the rear mounts may be formed usingthe adjustable suspension component 138 a or 138 b. In a Hotchkiss rearsuspension, lateral forces are applied at the front spring eye 181 andthe rear shackle attachment point 182. A front bushing 184 and a rearbushing 186 are used in the coupling. Leaf springs 188 extend betweenthe front spring eye and the rear shackle 182. The front spring eye andthe rear shackle 182 are coupled to the frame 190 of the vehicle.Bushings 184 and 186 are compliant bushings coupled to suspensioncontrol 88. By varying the signal from the suspension control to thebushings, the bushings are adjustable. By adjusting the compliance ofbushings, the amount of movement or articulation at the rear wheels maybe varied under lateral/longitudinal loading. That is, both wheels oneach side of the axle may be articulated relative to a vertical axiswhen in a turning mode. By controlling the movement of the suspensionthrough the bushings, the radius of curvature of the vehicle may also bereduced.

Another application of the invention is to utilize adjustable bushing in138 a and 138 b. Such a device may be hydraulically ormagneto-rheologically locked so that its attitude can be locked intocertain positions. When such a device, located at 138 a and 138 b, isunlocked and its attitude is compliant to longitudinal loads, abraking/traction force can induce an attitude change of the Hotchkisssuspension and its corresponding wheels. Upon favorable changes inplace, the attitude of Hotchkiss suspension and wheels is then locked bythe adjustable bushings 138 a and 138 b (either hydraulically ormagneto-rheologically). Effectively, the Hotchkiss suspension and wheelsare steered through both the braking/traction force and thelocking/unlocking of Bushings 138 a and 138 b. In other words, theHotchkiss suspension and wheels serve as a semi-active steering systemwhere the “semi-active” refers to the elimination of the need of a(usually costly) steering actuator in the said steering mechanism. Thesteering actuation as described above, is actually done by regulatingthe disturbances (i.e. longitudinal forces) coming into the steeringsystem.

Referring now to FIG. 7, a four link suspension is illustrated relativeto vehicle frame 190. The four link suspension includes a toe link 194.

Referring now to FIG. 8, toe link 194 is illustrated in further detail.Toe link 194 helps to control oversteer conditions that would be presentwith some rear independent suspensions. Because the suspension isslightly compliant, the toe link forces the outside rear wheel to toe-inslightly in a turn. Side loads in a turn presented at the toe link causethe toe link to articulate the lower control arm and toe in the outsidewheel. As is illustrated, a bushing 196 is used to couple the toe link194 to the body/frame 190. In a similar manner to that described withrespect to FIG. 6, the bushing 196 may be electrically controlled. Byelectrically controlling the bushing, the bushing may be made more orless compliant. In the present application, it may be desirable to makethe bushing more compliant during a turn so that more articulated wheelmovement is obtained. Also, the toe link may be adjustable much like amini-shock absorber. Such a device may be hydraulically locked ormagneto-rheological.

Another application of the invention is to utilize adjustable toe link194. Such a device may be hydraulically or magneto-rheologically lockedso that its length can be locked into certain positions. When theadjustable toe link is unlocked, its length can be adjusted throughbraking/ traction forces and the suspension geometry. Upon favorablelength is achieved, the adjustable toe link is then locked and thecorresponding wheel is steered to a new position due to the changedlength of toe link. Effectively, the adjustable toe links are steeringlinkages similar to tie-rods while the steering actuation is performedthrough the braking/traction forces and the suspension geometry. Inother words, the adjustable toe links serve as a semi-active steeringsystem where the “semi-active” refers to the elimination of the need ofa (usually costly) steering actuator in the said steering mechanism. Thesteering actuation as described above, is actually done by regulatingthe disturbances (i.e. longitudinal forces) coming into the steeringsystem.

Referring now to FIG. 9, an alternative vehicle 10′ is illustrated.Vehicle 10′ is an electric vehicle. The electric vehicle includeselectric motors M₁, M₂, M₃ and M₄. A throttle type input 197 is coupledto controller 198. Based on the throttle type input that generates athrottle type signal similar to that of an internal combustion engine,controller 198 controls the motors M₁-M₄. The throttle type input 197may for example, be a resistive-type pedal sensor or joystick. Thecontroller 198 is capable of independently controlling the torques ofthe individual motors. Thus, for example, a small torque may be providedat one wheel while a large torque is provided at the other wheel.Similarly, a negative torque may be provided at each of the wheels. Thatis, the motors may also generate a braking effect on the various wheels.Thus, to provide brake-steer, a differential torque, that is one largetorque and one small torque, may be provided on opposite wheels toobtain a brake steering effect. The motors may be operated using variousbatteries 199 as will be evident to those skilled in the art.

Referring now to FIG. 10, one method of operating the control system isillustrated. In step 207 the various sensors of the system aremonitored.

Monitoring the sensors 207 may include among other things, determining asteering wheel angle in step 208, determining a steering wheel directionin step 209, determining a steering wheel turn rate in step 210, anddetermining a steering wheel torque in step 211.

In step 212, the outputs of various vehicle systems are also monitored.Step 212 may also monitor the anti-lock braking system so that thewheels or wheel on which braking forces apply do not lock up. Thus, bymonitoring the wheel speeds, the wheels can be prevented from locking upand thus preventing tire wear at the particular wheel. The tractioncontrol system, yaw control system and/or rollover control system mayalso be monitored.

In step 214, whether the vehicle is in a parking mode is determined. Theparking mode may be determined by using various combinations of sensorssuch as the steering wheel angle sensor, the wheel speed sensor, thewheel speed direction, a combination of the wheel speed sensor and thesteering angle sensor, a driver actuated switch, the vehicle velocity,or a switch on the steering system (which may include a pressure reliefswitch or a limit switch). Another way in which to determine parking isusing a map stored in the controller memory 27 which correlates steeringwheel rate and vehicle velocity to a park or no-park condition. Thistype of map may be developed specifically for each vehicle duringvehicle development to correlate vehicle speed, steering wheel rate anda parking/non-parking condition.

Right before the vehicle gets into parking mode, the vehicle is likelyto be throttled. The vehicle may also be coasted in minimum speed,lightly braked if the driver is driving the vehicle in low speed. Thevehicle may be in or entering parking mode if the suspension heightsensor detects that the vehicle drives over speed bumps, if the visionsensor detects multiple still vehicles, if the throttle is reduced, ifthe driver brakes the vehicle from time to time, etc.

In the parking mode, if the steering wheel input is small, the vehiclemight be in a straight line parking condition. If the driver commandsthe steering wheel excessively in one direction, the control system maydetermine that the vehicle needs turning assist in the parking.

In step 216, vehicle normal force at each of the wheels and the staticloading of the vehicle may be adjusted. This step is an optional stepfor applying brake-steer. The suspension controls are used to adjustnormal forces by either open-loop modifying the normal forces ofindividual corners during brake-steering or closed-loop regulate thenormal forces by feeding back the estimated normal forces. For example,the controlled suspensions are adjusted so as to generate larger normalforces at braking/ driving wheels than the other wheels duringbrake-steer applications. The normal forces application can be applieddiagonally so that the vehicle attitude is not effected. Another type ofsuspension controls does not independently adjust the normal forces ofindividual corners but adjusts the weight distribution and/or weighttransfer. This normal force distribution helps improve theneutral-steer/over-steer characteristics of the vehicle and thereforehelps improve the turning radius of the vehicle at higher speeds. Totalvehicle loading or the normal load at each wheel may be determined. Thesuspension control and brake-steer control are also adaptively adjustedbased on the vehicle loading condition.

In step 218, brake-steer is applied to the vehicle. Step 218 maygenerate a steering enhance signal or other control signal based uponthe sensing of the desirability for brake-steer. For example, when thedriver selects a driver selectable mode, a steering enhance signal maybe desired if the vehicle is to be turned sharply. Thus, the steeringenhance signal may be used to reduce the turning radius of the vehicle.As mentioned above, brake-steer may take the form of applying brakes asin step 220, applying a positive torque in addition to applying brakesin step 222, or applying a differential torque. The differential torquemay be performed by providing one wheel with a greater positive torquethan a second wheel. This would be particularly useful in the electricvehicle 10′ described above. The transfer case mode or differentials mayalso be changed in a 4×4 vehicle from a 4×4 mode to a 4×2 mode toincrease control of the brake-steer. Proportioning brake-steer betweenfront and rear wheels may be performed by proportioning brakes betweenthe front and rear wheels. It should be also noted that applyingbrake-steer may be performed as a function of the steering wheel or handwheel torque as measured by the torque sensor 52. For example, moresteering wheel torque may correspond to a greater amount of brake-steerbeing applied. The torque applied in step 222 may also be applied as afunction of the traction control system. That is, the combination ofsteps 212 and step 222 may monitor the traction control system toprovide the proper amount of torque to the system to provide torque andto prevent wheel slip. That is, on a split mu surface as detected by thetraction control system or the mu sensor, a different amount of torquemay be required to be provided based on the coefficient of friction ofthe surface on which the particular wheel is on to prevent slip. In anyevent, the amount of torque to each wheel may thus be regulated.

Brake-steer may override the driver's braking request if the vehiclevelocity is already under a certain threshold, or the vision or camerasensor indicates that there is no danger of an obstacle. If the vehiclespeed is higher than another threshold, the brake-steer would be used togenerate differential brake in addition to the driver's braking.

Brake-steer might be overridden if the driver is requesting throttle orbraking beyond a certain threshold. In this case, the driver most likelywants to cease parking.

As mentioned above, it may be desirable to apply brakes to one wheel ofthe vehicle while applying a positive torque to the other wheel. Thisprevents the vehicle from stopping rather than continuing in a parkingcondition. By applying torque to the opposite wheel the turning radiusof the vehicle is also reduced. Brake-steer may also be performed basedon a calculation of the wheel speeds from each of the four wheels. Adesired wheel speed is calculated for the first wheel based upon thesecond wheel speed signal, the third wheel speed signal and the possiblefourth wheel speed signal. By calculating the desired first wheel speedsignal, braking and/or differential torquing may be applied to the firstwheel so that brake-steer is applied to the vehicle. Thus, bycontrolling the wheel speed, the turning radius of the vehicle may bereduced. In step 224, the normal load at selective wheel or wheels mightbe adjusted through suspension control or suspension modification. Thismay be done together with applying brake-steer in steps 220 or 222. Bymodifying the normal load of the suspension in step 224, the turningradius of the vehicle may be reduced further than brake-steer alone. Forexample, more normal load may be applied to one corner of the vehicle byraising and lowering the active suspension components. By placing morenormal load on a wheel that is braking, the turning radius may befurther reduced than that from brake-steer alone.

As mentioned above, the vehicle loading (total static loading or the lowfrequency portion of the sum of the normal forces at each wheel) may bea factor in the amount of brake-steer to apply. For example, morebrake-steer may need to be applied with a fully loaded (high payload)vehicle. Also, the amount of brake-steer may be modified based on theposition of the load, for example, if the vehicle has all the loadingaround the axle intended for brake-steer applications, the requiredbrake pressure for brake-steer may be reduced since the effective yawmoment is amplified . Loading and loading location detection may bedetermined directly using various suspension sensors or indirectly usingcalculations performed by other systems such as a yaw control and rollstability control system.

Throttle information may also be used in determining the amount ofbrake-steer to apply. For example, throttle information may be used toprovide a driving torque request, load estimation or the driver'sintention. Braking input may be used to override parking mode so thatall the brakes are applied to stop the vehicle.

Suspension modifications may also take the form of actively modifyingsuspension components such as those shown in FIGS. 6-8. A suspensioncontrol signal may be generated by the controller to change thecharacteristics of a particular wheel by articulating the wheel based onthe particular vehicle direction so that brake-steer may further enhancethe turning radius of the vehicle. One example above is a compliantcomponent of the Hotchkiss suspension. Another example is an adjustabletoe link in an independent suspension.

After steps 220, 222, and 224, pressure/torque feedback may be providedto the vehicle operator through the (hand) steering wheel in step 226.It should be noted that the amount of brake-steer may be coordinatedwith the amount of steering or steering wheel angle (of the hand wheel)provided by the vehicle operator. For example, up to a predeterminedthreshold, no brake-steer may be provided and after a predeterminedthreshold, a predetermined amount of brake-steer may be applied.

Another way in which steering feedback may be applied is that when thefront steering wheel 14 a reaches travel stops, an additional amount ofhand wheel steering wheel angle or torque may be used to control themagnitude of brake-steer. Reaching stops may be determined by pressureor limit switches. Thus, after a predetermined hand wheel anglecorresponding to the steering system travel stops, an amount of handwheel torque may correspond directly to an amount of brake-steer appliedto the vehicle. The amount of brake-steer on the vehicle may be reducedor increased based upon a combination of differential torque and/or theamount of braking applied to one or more wheels. Further, the amount ofbrake-steer may also be changed based upon a compliant suspensioncomponent or changing the normal load of the suspension.

Referring now to FIG. 11, a plot of various boost curves based upon SWArate versus brake steer is illustrated. In this example, the steeringwheel angle (SWA) is increasing toward the left of the plot. Asillustrated, the boost curves may be non-linear. Also, the boost curvemay be valid for vehicle velocities below a velocity threshold such as10 miles per hour. In FIG. 11, the steering wheel angle starts from theright side of the plot and increases leftward. As the steering wheelangle increases and reaches the brake steer entry threshold T₁,brake-steering is initiated. During the period from the brake-steeringentry threshold T₁ to P₁, the brake-steer gradually increases comparedto that from the period from P₁ to period P₂. The gradual increase isprovided to provide a smooth transition in the entry of brake-steer.From the period P₁ to period P₂ a larger slope and thus more aggressivebrake-steer is provided to more aggressively turn the vehicle. From theperiod P₁ to “Lock” the slope of the brake boost curve is reduced. Asillustrated, four boost curves B₁, B₂, B₃ and B₄ are illustrated. In theapplication of brake-steer one of the boost curves are followed unlessthe steering angle rate or steering torque increases or decreasesdescribed below. The determination of the boost curves may be based onthe steering wheel angle rate or the amount of torque applied to thesteering wheel. Thus, if the rate of turning of the steering wheel angleis greater, curve B₂, B₃ or B₄ may be chosen. The amount of increasingSWA rate or torque is illustrated by arrow 227. Thus, as the torque orsteering wheel rate increases, the slope, particularly in the areabetween periods P₁ and P₂, may be increased to more aggressively applybrake-steer to the vehicle. A linear plot of brake-steer is illustratedin dotted lines between brake-steer entry threshold T₁ and Lock. As canbe seen, the boost curves B₁-B₄ are more aggressive than the linear plotand thus provide more brake-steer earlier in the turn.

Preferably, a second set of boost curves is used when brake-steer is nolonger desired. That is, when the vehicle operator moves the steeringwheel angle in the direction toward center as opposed to away fromcenter in boost curves B₁-B₄, one of boost curves C₁-C₄ are followed. Asin the case above, the various boost curves are chosen based upon thesteering wheel angle rate or the steering wheel torque. More aggressivesteering wheel angle rate or steering wheel angle torque moves the boostcurves in the direction of arrow 229. Thus, at a lower SWA rate orsteering wheel torque, boost curve C₁ may be followed. Boost curve C₄may be followed upon the application of a high SWA rate or high steeringwheel angle torque. In the period between Lock and period P₃, a highnegative slope is applied to quickly exit brake-steer when brake-steeris no longer desired. Thus, the region between Lock and P₃ has anaggressive negative slope whereas the region between P₃ and brake entrythreshold has a non-linear curving characteristic.

Boost curve D₁ is provided to illustrate that when reaching the Lockposition is not reached, a non-linear boost curve is followed in thereverse direction. Thus, the boost curve D₁ starts on curve B₁ betweenperiod P₁ and P₂. The curve has a first portion that has an aggressivenegative slope that quickly removes brake-steer when the steering wheelangle travels in the direction toward center and tapers more positivelyas brake-steer entry threshold T₁ is approached.

Referring now to FIG. 12, the amount of brake-steer may be dependentupon an amount of torque alone. That is, as the amount of torqueincreases, the amount of brake-steer may not increase until abrake-steer threshold is achieved. Thus, an additional amount of torqueafter T₃ may increase the amount of brake-steer. For example, a linearfunction may be provided between brake-steer torque threshold T₃ and amaximum torque threshold T₄. Further, a boost curve similar to thatshown in FIG. 11 may be used. That is, a non-linear curve E₂ may beprovided in a forward direction that provides more aggressiveapplication of brake-steer. Thus, between the brake-steer torquethreshold T₃ and P₄ a gradual increase in the amount of brake-steer maybe provided. Between periods P₄ and P₅ an aggressive brake-steer may beprovided whereas in the period between P₅ and the maximum torquethreshold T₄ a lower or more gradual amount of brake-steer may beprovided. Upon reaching the maximum torque, when the vehicle torque isreduced, a second boost curve E₃ may be provided to quickly reduce theamount of brake-steer between period T₄ and P₅ and gradually reduce theamount of brake-steer between period P₅ and brake-steer torque thresholdT₃. Boost curve E₄ illustrates that if the amount of torque does notreach the maximum torque threshold T₄, a boost curve similar to that ofE₃ may be followed from any point on the plot E₂. Thus, the plots E₂ andE₃ are dependent upon the direction of the torque. It should be notedthat in FIGS. 11 and 12, the boost curves may be stored in a map in thememory 27 of FIG. 4.

FIG. 13 illustrates a low velocity scenario. If the vehicle velocity iszero and increases to a low velocity such as below two miles per hour,and the steering wheel angle is moved between a threshold T₅ andT_(max), this indicates that the vehicle operator may intend tobrake-steer the vehicle immediately from a parked position signaling atight turn. For example, this may signal a parking situation. Thus, whenthe vehicle is not moving, a maximum brake-steer may be immediatelyapplied. This brake-steer is applied fully since a jump in brake-steerwill not be perceived when the vehicle is not moving. Thus, as thevehicle starts to slowly move after being at zero velocity, maximumbrake-steer is applied to provide a maximum reduction in the steeringradius of the vehicle. Once the vehicle moves above a low vehiclevelocity such as two miles per hour, the boost curves of FIGS. 11 and 12may be used. This condition may also be dependent on other factors suchas a change of position of the shift lever, for example, from park toreverse.

Referring now to FIG. 14, another plot illustrating a boost curve at V=0to a very low velocity threshold such as two miles per hour isillustrated. In this example, as the steering wheel angle increasesbetween threshold T₆ and T₇, a gradual curve is applied but aggressivebrake-steer is applied when the plot reaches a maximum brake-steerbetween periods T₇ and T_(max). Of course, FIGS. 13 and 14 may alsoapply to a high steering wheel torque applied to the steering wheel. Ifthe steering wheel torque is applied above a threshold rate and thevehicle velocity is zero, when the vehicle starts to move rapid ormaximum brake-steer may be applied.

Referring now to FIGS. 11 and 15, the system may also apply brake-steerin a forward or a reverse vehicle direction with differing thresholds toprovide a different amount of brake-steer. In step 230, it is determinedwhether the vehicle is driven in a forward or reverse direction. Thereverse direction of the vehicle may be determined in several ways. Oneway in which the reverse direction may be obtained is determining adirection from a transmission shift lever. The shift lever may generatea reverse signal in a reverse position. A push button may also begenerated in a reverse direction. The reverse direction may also beobtained from other sources such as a transmission controller or a wheelspeed sensor.

In step 232, brake-steer is applied when predetermined conditions areabove a first threshold, for example, T₁ of FIG. 11 or T₃ of FIG. 12.When it is determined that the vehicle is in a reverse direction, instep 234, brake-steer conditions are applied when the vehicle is above asecond threshold T₁ “(FIG. 11) that may be different than the firstthreshold. The second threshold T₁” may be less than the firstthreshold. That is, the brake-steer may be applied earlier in reverse tobring about more benefits earlier than that of the forward position.Thus, brake-steer would be more easily or readily applied in a reversedirection. For example, the brake-steer may be applied starting at ahigher speed in the reverse condition than the forward condition. Also,in the reverse condition a lower steering wheel angle or steering wheeltorque may be used to actuate the brake-steer condition as illustratedby threshold T₁ “of FIG. 11. The whole boost curve plot may thus beshifted to the right. Thus, the second threshold may sensitize thesystem to apply brake-steer earlier. As illustrated in FIG. 12, a lowertorque threshold T₃ ” may be used to enter into brake-steer sooner.Also, brake-steer may be applied to different wheels from that in theforward direction. In the forward direction, brake-steer may be appliedto one of the rear wheels, while in the reverse direction brake-steermay be applied to one of the front wheels. Brakes are applied in step236. In step 238, a positive torque or differential torque may beapplied to one or more of the wheels. By applying a positive ordifferential torque to the wheels, brake-steer and/or brake-steerassistance may be obtained. As in the case of brakes, positive ordifferential torque may be applied to wheels opposite to those appliedin the forward direction.

In step 240, suspension modifications may be performed alone orsimultaneously with applying brakes or applying positive differentialtorque in step 238 as described above in FIG. 10.

In steps 236, 238, 240, the amount of brake-steer may be proportioned inresponse to a transfer case mode. That is, the transfer case of thevehicle may allow the amount of brake-steer to be proportioned betweenthe front and rear wheel. Further, the transfer case may also changemodes (e.g., from 4×4 to 4×2 mode) during brake-steer.

The system may then return to its original mode (e.g., 4×2 to 4×4 mode)automatically. The front, center and/or rear differentials may beswitched from locked to unlocked or vice versa. The amount ofproportioning may be varied depending on the vehicle driving conditions.

Referring now to FIG. 16, the wheels may be articulated in variousdirections based upon the direction and/or surface mu. A simplifiedversion of a vehicle illustrates the wheels 12 a, 12 b, 13 a, and 13 b.A compliant mount 200 having a locking mechanism 202 is illustrated. Thecompliant mount is mounted between rear wheels 13 a and 13 b. Lockingmechanism 202 may, for example, be a solenoid locking mechanism. Thesolenoid locking mechanism may allow one wheel to articulate relative tothe other wheel based upon the direction. For example, when traveling ina forward direction and the vehicle is desired to turn to the left, therear wheel 13 a may be articulated in the direction illustrated by arrow204 a. When the vehicle is desired to turn in a right direction, lockingmechanism 202 may allow wheel 13 b to articulate in the directionillustrated by arrow 204 b.

Referring now to FIG. 17, when the vehicle is turning in a rearwarddirection, the desired vehicle direction may be opposite of that shownin FIG. 16. For example, in a rearward direction to turn the rear of thevehicle toward the right side of the vehicle (relative to the vehicletraveling in a forward direction), the rear wheel may be articulatedoutward in the direction shown by arrow 204 c. In the rearward directionwhen the vehicle is to be driven to the left in a rearward direction(relative to the forward direction of the vehicle), the wheel 13 b isarticulated in the direction shown by arrow 204 d.

Referring now to FIG. 18, on a split mu surface having a low mu surface205 and a surface 206 having a mu higher than that of 205, if thevehicle is traveling in a forward direction and is desired to turn in aleft direction, wheel 13 b is articulated in the direction shown byarrow 204 e. The directions illustrated by arrows 204 a-204 e reduce theturning radius of the vehicle.

Referring now to FIGS. 19 and 20, the present invention may also be usedto enhance the trailerability of a vehicle.

In step 250, it is determined if the vehicle is trailering.

The presence of a trailer may be determined in several manners,including, for example, the hitch sensor, the reverse aid system, anultrasonic sensor (which may be one of the reverse aid system sensors),monitoring the current through a harness, a push button, a camera, oralgorithm-based loading or loading detection through existing vehicledynamics sensors. The algorithm-based vehicle loading and loadinglocation determination uses the vehicle dynamics control sensor sets. Ifthe system determines a large loading and loading location significantlybeyond the rear axle, the vehicle has a trailer.

In step 252, the vehicle systems are monitored and/or adjusted. In step254 the various vehicle sensors are monitored. When it is determinedthat the vehicle is operating in a reverse direction which may beperformed in a similar manner to that described above with respect tostep 230, step 256 is executed. After step 256, the steering wheel angleinput 258 from the plurality of sensors is input to the system. In step260 and as illustrated in FIG. 20, a predicted path is displayed inresponse to the present position and the predicted position based uponthe steering input. In step 260, the current position of the trailer 160relative to the vehicle may be displayed on display 68. As well, variouspredicted positions may also be displayed. The positions may bedetermined as a function of the current steering wheel angle and thecurrent angle between the trailer and vehicle. Interference I₁ betweenthe trailer 160 and vehicle 10 may also be displayed or highlighted socorrective actions may be performed by the vehicle operators. Videocameras may be mounted as high as possible on the trailer to determinethe relative positions of the vehicle and trailer. The system would becalibrated for the specific trailer; once calibrated, no furtheradjustment would be needed. Specifically, camera height, the distancebetween hitch and trailer wheels, and the critical vehicle-trailer angleat which interference occurs may be needed. These parameters could beprovided to the controller by the driver prior to use. In thisembodiment, no measurement of trailer-vehicle angle would be needed.

Alternatively, if sensors were available to measure the trailer-vehicleangle, much of the calibration procedure may be made automatic. Thetrailer-vehicle angle may be measured in any of a number of ways: viathe vehicle's backup ultrasonic backup sensors, by load cells on thehitch post, or by a mechanical means such as a retractable cable. Tocalibrate the system, the driver would back up to and deliberatelyalmost jackknife the trailer. At that point he would tell the system bya type of input device that this was the critical angle. As the driverpulled forward again, the change in trailer-vehicle angle as a functionof forward distance traveled could be automatically measured and used tocalculate the hitch-wheel distance.

Two, three, four or five different predictions may be displayed basedupon the current steering conditions. In FIG. 20, the current positionX₁, and predicted positions X₂ and X₃ are illustrated. This will give anindication to allow the vehicle operator to correct the position of thevehicle. Also, the vehicle speed may be used as input to determine thedisplay.

In step 262, brake-steer may be generated based upon the input. Itshould be also noted that the turning input in step 258 may be providedby the steering device or may be provided by a push button, a turnsignal lever or other type of device.

Referring back to step 254, if the vehicle is turning in a forwarddirection, brake-steer may also be generated in step 262.

If the vehicle is in a forward direction in step 264, an additional step265 may be performed before step 262. In step 265, the yaw rate desiredby the vehicle operator may be determined from the hand wheel andcompared to the yaw rate from the yaw rate sensor. If the yaw rate fromthe hand wheel varies from the yaw rate from the yaw rate sensor (whichindicates that the driver's intent is not being followed), thenbrake-steer may be applied in step 262.

Referring back to step 254, if the vehicle is in a straight forwarddirection high speed condition in step 266, various conditions relatedto the vehicle may be determined such as the rear axle side slip anglein step 268. Also, a profile may be obtained in step 270 of the trailerbehind the vehicle. That is, a camera or reverse sensing system maygenerate an electronic profile that indicates the vehicle is moving in astraight ahead stable condition.

Thus, in step 262, brake-steer may be generated with the trailer brakes,the vehicle brakes, vehicle suspension changes, or a combination toassist the vehicle in the reverse condition 256, turning in a forwarddirection from step 264, and in response to high speed straightcondition from steps 266-270. Brake-steer may be provided by activatingvehicle brakes in step 274, applying positive or differential torque tothe wheels of the vehicle in step 276, activating the trailer brakes 278or other combinations mentioned above. All or some of the steps 274-278may be performed simultaneously. In addition, the vehicle loading and/orsuspension position may be changed in step 280. Thus, brake-steer may beused to enhance the straight ahead high speed trailerability of thevehicle, a turning forward direction of the vehicle, and in a reversedirection of the vehicle. The straight ahead condition may be enhancedby brake-steer to a lesser extent than a turning mode.

In step 268, the rear axle side slip angle of the vehicle may beestimated and monitored. When the rear axle side slip angle is above apredetermined value together with its rate change above a certainthreshold (indicating that the side slip angle is constantly crossingzero), the vehicle velocity is above a velocity threshold, and thesteering wheel is about zero, and the brake-steer system determines thatthe vehicle is in straight line driving and the trailering ispotentially unstable, brake-steer is applied to the vehicle. Notice thatthe controlled brake torque may be set to be proportional to themagnitude of the rear axle side slip angle and/or the magnitude of therate change of the rear axle side slip angle.

Notice that in straight line driving, the vehicle yaw stability controlusually does not activate. Hence, the brake-steer action for straightline and unstable trailering is in addition to the yaw stabilitycontrol.

In step 264 a turning and trailering condition is determined. Thebrake-steer is activated upon the detection of a potentially unstabletrailering condition. The yaw stability control tuned for normal vehicledriving is based on the driver's intention to control the over-steer orunder-steer of the vehicle such that the vehicle is maintained on thedriver's intended course. Since yaw error feedback and side slipfeedback are used in yaw stability control, they can cause problemsduring turning and unstable trailering. In one aspect, the divergenttrailer lateral motion would cause a fluctuation of the vehicle yaw rateand side slip angle. From yaw stability control point of view, thevehicle crosses the over-steer and under-steer boundary from time totime. Hence under-steer correction (tries to make car steer more) andover-steer control (tries to make car steer less) will activate. Ifthose activations are not carefully done, the trailer's dynamic lateraldeviation may be excited instead of controlling the trailering.

Therefore it is desirable to provide a control system to enhance thetraditional yaw stability control upon the detection of the unstabletrailering during a turning. Such a system uses the braking to steer thevehicle in an opposite direction of the trailer motion so as tostabilize the trailering. Such a system includes determining a presenceof a trailer, determining a vehicle velocity, determining a hand wheelangle position signal of the hand wheel, determining a sensor yaw ratefrom the yaw rate sensor, calculating a desired yaw rate based upon thehand wheel angle position signal (which reflects the driver'sintention), determining a rear axle side slip angle, and controlling thebrakes of the vehicle such that the vehicle unstable trailering iseliminated.

More specifically, upon the detection of trailering in a turned vehicle,when the rear axle side slip angle is determined to be above apredetermined rear axle slip with rate change of the side slip angleabove certain threshold, and the vehicle velocity is above a vehiclevelocity threshold, one or more brake control commands (amount of thebrake pressures) are generated based on the magnitude of the calculatedrear side slip angle and the magnitude of its rate change, the yawangular rate, and the desired yaw angular rate.

The location of brakes in which the braking pressures are sent isdetermined based on a simple role of thumb: to reduce the magnitude ofthe rear side slip angle. That is, when there is a positive rear sideslip angle, the braking is applied to a wheel such that the vehicleintends to generate negative side slip angle upon the application ofbraking; when there is a negative rear side slip angle, the braking isapplied to a wheel such that the vehicle intends to generate positiverear side slip angle upon the application of braking.

It should also be noted that the reversing direction may also beenhanced using the straight ahead condition. That is, the profiledetermined in step 270 may be used to maintain the trailer straightbehind the vehicle if so desired. Such an input may be provided by theoperator of the vehicle through the hand wheel or through a push button.Thus, after the vehicle has gone straight ahead for enough time toobtain a profile, brake-steer may be applied to the trailer and/orvehicle to maintain the trailer straight behind the vehicle untilturning is desired. Thereafter, when it is determined that turning isdesired, steps 256-262 may be performed.

Referring now to FIG. 21, the present invention may be used to assist avehicle in a U-turn condition. In step 300, a U-turn is detected fromthe various sensors and/or inputs. For example, a push button on theinstrument panel or one of the levers may be provided to assist ortrigger the vehicle into an assist mode based upon a U-turn. U-turns mayalso be sensed. Detecting a U-turn may be formed from a steering wheelangle sensor, the wheel speed sensors, the yaw rate sensor, vehiclevelocity sensor, throttle position sensor, and yaw rate sensors, orvarious other combinations of the sensors described above. For example,from the sensors a straight path followed by a sharp turn (SWA, SWA rateor combination) and a reduction in speed or possible stop may be aninitial condition before entering a U-turn. Thereafter, an increase inspeed with the steering wheel turned may indicate the vehicle is in aU-turn.

In step 302, brake-steer is activated in response to detecting theU-turn signal. Brake-steer may be maintained until a threshold isexceeded. For example, the velocity threshold may be 18 miles per hour.Brake-steer may be applied in a similar manner to FIGS. 11 and 12 exceptthat a higher speed threshold may be used to allow for a higher speedturn. The U-turn signal may be generated within the controller.

In steps 304-310, various ways to provide brake-steer are set forth. Instep 304, the vehicle brakes may be activated in order to generatebrake-steer. In step 306, positive or differential torque may be appliedto the vehicle to provide brake-steer. Also, if the vehicle is towing atrailer, the trailer brakes may be activated in step 308 to providebrake-steer. Further, in step 310, the suspension may be adjusted toperform brake-steer. The suspension modifications may take the form ofshifting the normal loads, for example, to the appropriate wheel orarticulating the active or adjustable suspension components 136 a, 136b, 138 a, 138 b.

Referring now to FIG. 22, the present invention may also be used toassist in object avoidance. In step 340, the various vehicle sensors aremonitored. In step 342, the object detection system is monitored. Instep 344, the distance to an object may be determined by the objectdetection system. As mentioned above, the object detection system maycomprise the lidar, radar, sonar and cameras described above. In step346, the controller determines the likelihood and impact based onvarious conditions including the object distance, relative speed,direction of the object and vehicle, etc. If there is little or nolikelihood of an impact, step 348 is executed in which nothing is done.In step 346, if an impact is likely, step 350 is executed. In step 350,if impact is likely, the amount of braking may be provided in proportionto the distance to the object in step 352. For example, the closer theobject, the brake, brake-steer or combinations thereof may be increasedsubject to prioritization constrains. In step 354, the distance to theobject is continually monitored. In step 356, an amount of brake-steermay be applied to one or more of the wheels to assist the vehicle tosteer away from the object to avoid a collision. Brake-steer may beapplied as in the above methods by activating the brakes, applyingpositive or differential torque, activating trailer brakes or adjustingthe suspension actively or providing a load shift to the vehicle. Thus,the brake-steer may provide a supplemental braking to the proportionalbraking described above.

It should also be noted that during brake-steer it may be desirable toprioritize the system so that upon driven braking the system exits orceases brake-steering. Further vehicle speeds above a threshold orsignificant driver caused acceleration may cause brake-steer to ceasebeing applied.

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. A method of controlling a vehicle with a trailer comprising:determining a presence of the trailer; determining a vehicle velocity;determining a steering wheel angle; determining a rear axle side slipangle of the vehicle; and applying brake-steer to the vehicle when therear axle slip angle is above a predetermined rear axle slip angle, thevehicle velocity is above a velocity threshold, and the steering wheelangle is about zero.
 2. A method as recited in claim 1 whereindetermining the presence of a trailer comprises determining the presenceof a trailer with a hitch sensor.
 3. method as recited in claim 1wherein determining the presence of a trailer comprises determining thepresence of a trailer with a reverse aid sensor.
 4. A method as recitedin claim 1 wherein determining the presence of a trailer comprisesdetermining the presence of a trailer with an ultrasonic sensor.
 5. Amethod as recited in claim 1 wherein determining the presence of atrailer comprises determining the presence of a trailer with a camera.6. A method as recited in claim 1 wherein determining the presence of atrailer comprises detecting a locating plate behind the vehicle.
 7. Amethod as recited in claim 6 wherein the locating plate comprises alocating hole positioned along the trailer tongue.
 8. A method asrecited in claim 1 wherein applying brake-steer comprises applying atleast one brake at a first wheel to reduce a vehicle turning radius. 9.A control system for an automotive vehicle and a trailer comprising:means to determine the presence of the trailer; a vehicle velocitysensor generating a vehicle velocity signal; a steering wheel anglesensor generating a steering wheel angle signal; and a controllercoupled to the means, the velocity sensor and the steering angle sensor,said controller determining a rear axle side slip angle of the vehicle,and when the rear axle slip is above a predetermined rear axle slip,vehicle velocity is above a velocity threshold and the steering wheelangle is about zero, said controller programmed to apply brake-steer tothe vehicle.
 10. A system as recited in claim 9 wherein said means todetermine the presence of a trailer comprises a hitch sensor.
 11. Asystem as recited in claim 9 wherein said means to determine thepresence of a trailer comprises a reverse aid sensor.
 12. A system asrecited in claim 9 wherein said means to determine the presence of atrailer comprises an ultrasonic sensor.
 13. A system as recited in claim9 wherein said means to determine the presence of a trailer comprises acamera.
 14. A system as recited in claim 9 wherein said controller isprogrammed to brake-steer by applying a first brake and a second braketo reduce the turning radius of the vehicle.
 15. A system as recited inclaim 9 wherein said controller is programmed to brake-steer by applyingat least one brake at a first wheel to reduce a vehicle turning radius.16. A system as recited in claim 9 wherein said controller is programmedto brake-steer by applying an increased drive torque to a second wheelrelative to the first wheel.
 17. A control system as recited in claim 9further comprising a steering wheel angle sensor generating a steeringwheel angle signal, said controller programmed to apply brake-steer inresponse to a reverse direction signal and the steering wheel anglesignal.
 18. A control system as recited in claim 9 further comprising ayaw rate sensor generating a yaw rate signal, said controller programmedto apply brake-steer in response to a reverse direction signal and yawrate signal.
 19. A control system as recited in claim 9 furthercomprising a steering wheel torque sensor generating a steering torquesignal, said controller programmed to apply brake-steer in response to areverse direction signal and steering torque signal.
 20. A controlsystem as recited in claim 9 further comprising a steering wheel anglesensor generating a steering wheel angle signal and a vehicle velocitysensor generating a vehicle velocity signal, said controller programmedto apply brake-steer in response to the reverse direction signal andsteering wheel angle and vehicle velocity signal.
 21. A method ofcontrolling a vehicle with a trailer comprising: determining a presenceof the trailer; determining a vehicle velocity; determining a hand wheelangle position signal corresponding to an angle of the hand wheel angleposition; determining a sensor yaw rate from a yaw rate sensor;calculating a yaw rate based upon the hand wheel signal; determining arear axle side slip angle; and applying brake-steer to the vehicle whenthe rear axle slip angle is above a predetermined rear axle slip angle,the vehicle velocity is above a velocity threshold, and the sensor yawrate is diverging from the hand wheel yaw rate.
 22. A method as recitedin claim 21 wherein determining the presence of a trailer comprisesdetermining the presence of a trailer with a hitch sensor.
 23. A methodas recited in claim 21 wherein determining the presence of a trailercomprises determining the presence of a trailer with a reverse aidsensor.
 24. A method as recited in claim 21 wherein determining thepresence of a trailer comprises determining the presence of a trailerwith an ultrasonic sensor.
 25. A method as recited in claim 21 whereindetermining the presence of a trailer comprises determining the presenceof a trailer with a camera.
 26. A method as recited in claim 21 whereinapplying brake-steer comprises applying at least one brake at a firstwheel to reduce a vehicle turning radius.